Air drying systems are well known and practiced in a variety of technical fields. One such prior art air drying system is a single tower system illustrated in FIG. 1 of U.S. Pat. No. 5,423,129. Shown herein as prior art FIG. 1, the prior art single tower air drying system is designed to provide clean and dry compressed air to a pneumatic system such as a brake system of a railroad train. The prior art system accomplishes this by removing moisture and airborne particulates from a stream of compressed air as it passes through a desiccant material contained within the single tower.
FIG. 1 illustrates a cross-sectional view of the prior art system. From right to left FIG. 1 shows an opening through which unpurified compressed air is received; a sump volume; the single tower housing the desiccant material; a purge check valve with a choke; a side chamber connected to a purge volume; a discharge air filter element; a discharge check valve; and an output chamber through which purified compressed air passes eventually to the pneumatic system.
In operation, the prior art air drying system receives from an air compressor (not shown) a supply of air which typically contains an unacceptably high amount of moisture and other particulates suspended therein. This unpurified compressed air passes into the sump volume and then flows upwardly eventually reaching the desiccant material. The desiccant plays the key role within the single tower air drying system in that it absorbs the moisture and traps various particulates (e.g., dust, dirt, etc.) as the compressed air moves radially into and through the desiccant material. Once moisture and particulates are extracted from the air stream, the cleaned and dried compressed air continues flowing from the center of the desiccant material through the purge check valve situated near the top of the single tower. This purified compressed air then passes through the side chamber eventually reaching the purge volume identified by reference numeral 5 in FIG. 1.
The purge volume of the prior art air drying system is capable of holding approximately five-hundred cubic inches (500 in.sup.3) of purified compressed air. When the air compressor is cycled off, the single tower system operates in a purge mode. During the purge mode, the purified pressurized air contained within the purge volume passes slowly in the reverse direction through the choke in the purge check valve and then back through the desiccant material. This slow stream of dried air reabsorbs a portion of the moisture previously collected within the desiccant material. Having evaporated in this passing stream of dry air, the evaporated moisture eventually exhausts through the sump volume to atmosphere. This gradual purging of dry air back through the system serves to dry out and thus rejuvenate the desiccant material. When the air compressor is again cycled on, the single tower system operates in a drying mode. During the drying mode, the desiccant material then again removes moisture from the stream of unpurified compressed air passing therethrough.
There are, however, several disadvantages inherent to the prior art drying system. Perhaps the most apparent disadvantage is that the source of unpurified compressed air must be cycled off in order to purge the desiccant material of the moisture it has accumulated. This serves to deprive temporarily the pneumatic system of a steady supply of clean and dried compressed air while the compressor is turned off. This shortcoming can prove quite inconvenient in certain applications.
Another disadvantage of the single tower air drying system is that it is only capable of removing a certain amount of moisture during the purge mode. Because the volume of unpurified air flowing into the prior art system vastly exceeds the volume of purified air used to purge the desiccant material, the desiccant material never adequately exsiccates during operation of the single tower system. Apparently, the desiccant material adequately exsiccates only after the prior art system has been turned off for a time sufficient to accomplish same.
The present document, however, discloses a switching and purging mechanism for a twin tower air drying system that overcomes the disadvantages of the prior art single tower system. First, the twin tower system need not cycle off the source of unpurified air to purge the air drying system of accumulated moisture. The prior art system, however, does. Second, the twin tower system more efficiently exsiccates the air stream than does the prior art single tower system.
Regarding the first advantage, the twin tower system continuously supplies purified air to the pneumatic system to which it is attached. Specifically, while one drying assembly of the twin tower system operates in the drying mode and therein supplies dry air to the pneumatic system, the other drying assembly operates in the purge mode and is therein purged of moisture it previously accumulated. After a predetermined time, the twin tower system switches the latter drying assembly to the drying mode and the former drying assembly to the purge mode. This switching continues until the source of pressurized air ceases supplying unpurified air to the twin tower system. Unlike the prior art system, the instant system need not deprive the pneumatic system of a steady supply of clean and dried compressed air while purging itself of moisture.
Regarding the second advantage, the switching of the two drying assemblies alternately between the drying and the purging modes allows the twin tower system to exsiccate the air stream more efficiently than the prior art single tower system. Two desiccant towers rather than one are employed in the air drying system with one absorbing moisture while the other is being purged of it. The switching of the two drying assemblies alternately between the drying and the purging modes thus serves to continuously purge moisture from the twin tower system. More fully desiccated air is thus supplied to the pneumatic system. The amount, density and overall surface area of the desiccant can also be selected to suit varying needs.
The twin tower system can be applied to a wide variety of pneumatic systems. Typical of the types of pneumatic systems to which the twin tower system could be applied include the pneumatic brake systems of passenger and freight railroad trains, subway trains and various other types of rail related transportation systems. Further examples include the pneumatic brake systems of various truck transport vehicles. Other types of pneumatic systems to which the twin tower system could be applied may be found outside the transportation field.
The foregoing background information is provided to assist the reader in understanding the instant invention and any terms of art used herein are not intended to be limited to any specific meaning unless specifically stated otherwise in this specification including the following detailed description.